Nonwoven fabrics with a marked 3D structure are important in technical textile applications such as geotextiles, automotive components, upholstery and foam replacement, insulation (thermal and sound insulation applications) and civil engineering amongst others. A fabric with high compressive force resistance and high longitudinal and transverse load-bearing capacity is described by Seegar et al (2000). This fabric was manufactured using a modified stitch-bonding machine of the Malimo type. A maximum tensile strength of 150 KN/m was obtained with a fabric weight in the range 500-1500 g/m2 and a production rate up to 5 m/min was claimed. The fabric was intended for use in drainage applications, barrier and insulation products. U.S. Pat. No. 5,906,879 describes a 3D thermal-bonded nonwoven fabric made of a bulky layer comprising a highly crimped conjugated bi-component fibre with a plurality of peaks separated by channels. This structure has high resiliency and excellent absorbency. The web was formed using, either spunbonding, carding, air-laying or wet-laying followed by thermal bonding and fabric weights are in range 15-240 g/m2. Applications are in personal-care absorbent products. An air-lay system to produce 3D webs and shell structures from staple fibre has also been reported, (Gong et al 2000) using porous moulds on to which fibres are deposited. The web is consolidated using through air bonding. Other established methods for forming 3D nonwovens rely on orienting fibres perpendicular to the fabric surface, including high-loft air-lay technology (Lennox-Kerr 1998) and perpendicular-laid structures (Ward 2000). There are also methods involving the deposition of fibres on a contoured collecting surface (for example U.S. Pat. No. 6,146,580, U.S. Pat. No. 5,575,874, U.S. Pat. No. 5,853,628, U.S. Pat. No. 4,741,941, U.S. Pat. No. 4,103,058) followed by bonding. In contrast to normal 3D nonwoven fabrics the formation of nonwoven fabrics with discrete voids or cells within the cross-section of the structure is a further important variation.
In U.S. Pat. No. 5,475,904, Le Roy describes a method for producing 3D structures by joining two or three fibrous materials together with a space or void left between the basic layers. The layers of fibrous materials can be woven, knitted, nonwoven or a combination of these. Barbed needles operating between two stripper plates transfer fibres from one layer to another to form links or bridges between separate layers. Alternatively, it is claimed that joining the layers can be achieved by stitching or ultrasonic welding. The two layers are kept a predetermined distance apart by a spacer plate. The spacer plate and stripper plates are adjustable by hand wheels and allow structures ranging in thickness from 5 to 50 mm to be made. Filling materials can be introduced between the two basic layers, which may be resin, powder, fibres, tubes, wire, threads, and/or electrical conductors. It is claimed that various different structures can be more economically formed using this approach compared to conventional methods. The 3D materials produced can be used in drainage, reinforcement, and insulation applications.
Le Roy also describes the use of needlepunching to interconnect layers. However, this approach suffers from the disadvantage that the speed of production is limited to a maximum of about 10 m/min and generally there are limitations in simultaneously bonding and connecting layers in lightweight fabrics below 100 g/m2. Thus, a preconsolidated web structure is normally required which tends to increase the cost of production. A further disadvantage of the method of the prior art is the risk of needle breakage, which has a deleterious effect on fabric quality and product acceptance in critical applications (e.g. contact layers used in woundcare).
The bonding technology that is adapted by the prior art (e.g. needlepunching) does not permit light-weight 3D nonwoven spacer fabric structures to be produced and the fabrics of the prior art are comparatively thick. The dimensions and geometry of the voids are also limited.